Heavy metals are useful as catalysts in many industrial processes. In many of these processes, a portion of the heavy metals may enter into waste streams, such was wastewater streams. Recovery of the heavy metals from these waste streams is often desirable so that the heavy metal may be reused as a catalyst. In other applications, it may be desirable to remove the heavy metals from waste streams before the waste streams are released to the environment.
In one particular known industrial process, aromatic carboxylic acids are made by liquid-phase oxidation of aromatic feedstock in an aqueous acetic acid solvent in the presence of air and a heavy metal catalyst. The heavy metal catalyst may be, for example, a bromine promoted catalyst that comprises cobalt and manganese. The oxidation yields aromatic carboxylic acid together with by-products, including partial and intermediate oxidation products of the aromatic feedstock, and acetic acid reaction product. Water is also generated as a by-product in the reaction.
The aromatic carboxylic acid produced in the oxidation process is withdrawn from the reactor as a slurry of crystals in a mother liquor comprising mainly aromatic carboxylic acid together with water, organics and dissolved catalyst components (cobalt and manganese in the form of their acetates). In typical known processes, after separation of the aromatic carboxylic acid product from the mother liquor of the slurry, a major part of the mother liquor and its catalyst heavy metal content are recycled to the oxidation reactor, and the minor part is purged from the system to avoid undue build-up of organic contaminants within the reaction system. The mother liquor purge is treated to recover acetic acid for recycle to the oxidation reaction, precipitate the catalyst metals as carbonates or bicarbonates so that they can then be recovered for further treatment, if necessary, and recycled to the oxidation reactor.
Consequently, there is always a need to recover heavy metals more efficiently for reuse in a range of industrial processes.